The present invention relates to the transfer of material between objects in relative motion, and more particularly to a control system for regulating the application of a solid or liquid material to a moving target.
The application of liquid or solid material to a target in relative motion includes applying ink, paint, or adhesives to a substrate; applying cleaning fluids, or applying decorative edible materials such as icing, sugar, or batter.
Traditionally the application of the material to a target is signaled to commence by an electrical impulse from a sensor (e.g., a photoelectric xe2x80x9ceyexe2x80x9d). The impulse from the sensor is transmitted to a dispenser. These devices are generally divided into two main categories: (i) those that actuate the dispenser after a time delay to locate the point of application on the target (time delay systems) and (ii) those that utilize velocity or location information from an external sensor to control the moment of application (velocity/position sensor systems).
Time delay systems can be operator adjusted to position the point of application on the target. Velocity/position sensor systems can be used over a relatively wide range of conditions compared to the time delay systems, but the velocity/position sensor systems require a somewhat more complex control method utilizing velocity (or position) information from the external sensor.
In the velocity/position sensor systems, the greatest difficulties arise because of the need to compensate for the time delay between the instant that a control unit or sensor actuates the dispenser, by transmitting a signal to it, and the instant that the material released by the dispenser actually contacts the target. The delay between the instant that the control unit issues an initiate or open command and the instant that the material makes contact with the target is often referred to as xe2x80x9cdelivery delayxe2x80x9d. Delivery Delay can be reduced, but not eliminated, thus always resulting in some placement error. Accuracy in placement of the application material is at least partially dependent upon the delivery delay. The placement error resulting from delivery delay increases as the relative target speed increases.
Most velocity/position sensor systems can compensate for delivery delay under steady-state velocity conditions. Thus, if the targets are moving at a constant velocity, the effects of delivery delay can be reduced by using mathematical calculations to xe2x80x9cfactor inxe2x80x9d the dispenser delay interval and material time-of-flight, and advance the moment of activation of the dispenser by an amount which approximates the delay interval.
One approach for compensating for delivery delay entails decreasing the size of the photo detection area of a photoeye. This approach requires that calculations be done much more frequently to account for variations in the conveyor speed. Thus, there would be a need for more frequent inputs to the control logic which controls the dispenser.
Delivery delay is not well compensated by the prior art devices which rely on the simple mathematical model of geometry. Because the timing of prior devices is based on a tachometer measurement of conveyor speed, delivery delay could only be done to the nearest range cell, the smallest unit measured. At higher conveyor speeds, the range cells must be made larger to avoid exceeding the computational bandwidth of the control unit, resulting in a potentially noticeable error in the placement of a bead under conditions of changing conveyor speed. Because an operator-adjustable multiplier is used, the dispensed material tends to fall between two range cells. Since microprocessors in the controllers had other tasks, any appreciable change in conveyor speed would introduce an error in bead position. Adjustments could only be made once per item, therefore, long bead applications would vary greatly from its desired position due to changes in conveyor speed.
Therefore, the need exists for a method and apparatus for selectively applying material to a target, wherein delivery delay is inherently accommodated. A need also exists for a system that can adjust to varying target speeds. The need exists for a simple technique to accurately place material on a moving target wherein the target speed may have some variation and the target size may be variable and relatively small.
The present invention includes an apparatus for selectively applying a material to a target moving along a path and includes a detector for identifying position of the target along the path; a dispenser spaced from the detector by an offset distance, the dispenser configured for selectively releasing material to contact the target; a velocity sensor for creating a velocity signal proportional to a velocity of the target; and a controller for selectively varying the offset distance in response to the velocity signal.
In a particular embodiment, an actuator moves the detector in a direction opposite to the direction of conveyor travel as the conveyor speed increases and moves the detector in the same direction as the conveyor travel as the conveyor speed is decreases. In a further configuration, multiple position detectors may be employed to provide further control signals in the application of the material.
The present invention addresses at least three separate sources of delivery delay in a typical application system:
1. Valve motion delay, resulting from the moving elements of the valve changing position at finite speed. For example, if mechanical solenoids are employed, the magnetic fields generated within the solenoids require appreciable amounts of time to build up when the valve is energized or decay when the valve is de-energized;
2. Time of flight delay resulting from the finite speed at which the material is propelled across the distance between the valve/nozzle assembly and the target location; and
3. Column inertia resulting from the mass of the column of delivery material resident in a delivery material supply, and the viscosity of the material. Because of these physical properties, the column requires some finite amount of time to get moving when the open command is sent to the valve. This delay is influenced by the delivery pressure and temperature.
The present invention accommodates changes in the conveyor speed and reduced spacing between targets. The present device also offers the advantages of:
1. employing physical geometry to accomplish what is previously accomplished by means of computational power as through a microprocessor in a control unit;
2. compensating for acceleration of target items at rates far higher than can be achieved by present methods based on digital control techniques; and
3. controlling the deposition of material with position resolution considerably greater than can be achieved through prior devices.